Expression of the apical iodide transporter in human thyroid tissues: a comparison study with other iodide transporters

Analytical and therapeutic profiles of DNA methylation alterations in cancer; an overview of changes in chromatin arrangement and alterations in histone surfaces

DNA methylation is the most important epigenetic element that activates the inhibition of gene transcription and is included in the pathogenesis of all types of malignancies. Remarkably, the effectors of DNA methylation are DNMTs (DNA methyltransferases) that catalyze de novo or keep methylation of hemimethylated DNA after the DNA replication process. DNA methylation structures in cancer are altered, with three procedures by which DNA methylation helps cancer development which are including direct mutagenesis, hypomethylation of the cancer genome, and also focal hypermethylation of the promoters of TSGs (tumor suppressor genes). Conspicuously, DNA methylation, nucleosome remodeling, RNA-mediated targeting, and histone modification balance modulate many biological activities that are essential and indispensable to the genesis of cancer and also can impact many epigenetic changes including DNA methylation and histone modifications as well as adjusting of non-coding miRNAs expression in prevention and treatment of many cancers. Epigenetics points to heritable modifications in gene expression that do not comprise alterations in the DNA sequence. The nucleosome is the basic unit of chromatin, consisting of 147 base pairs (bp) of DNA bound around a histone octamer comprised of one H3/H4 tetramer and two H2A/H2B dimers. DNA methylation is preferentially distributed over nucleosome regions and is less increased over flanking nucleosome-depleted DNA, implying a connection between nucleosome positioning and DNA methylation. In carcinogenesis, aberrations in the epigenome may also include in the progression of drug resistance. In this report, we report the rudimentary notes behind these epigenetic signaling pathways and emphasize the proofs recommending that their misregulation can conclude in cancer. These findings in conjunction with the promising preclinical and clinical consequences observed with epigenetic drugs against chromatin regulators, confirm the important role of epigenetics in cancer therapy.

Repeated Low High-Density Lipoprotein Cholesterol and the Risk of Thyroid Cancer: A Nationwide Population- Based Study in Korea

BACKGROUND

High-density lipoprotein cholesterol (HDL-C) plays an important role in the reverse cholesterol transport pathway and prevents atherosclerosis-mediated disease. It has also been suggested that HDL-C may be a protective factor against cancer. However, an inverse correlation between HDL-C and cancer has not been established, and few studies have explored thyroid cancer.

METHODS

The study participants received health checkups provided by the Korean National Health Insurance Service from 2009 to 2013 and were followed until 2019. Considering the variability of serum HDL-C level, low HDL-C level was analyzed by grouping based on four consecutive health checkups. The data analysis was performed using univariate and multivariate Cox proportional hazard regression models.

RESULTS

A total of 3,134,278 total study participants, thyroid cancer occurred in 16,129. In the crude model, the hazard ratios for the association between repeatedly measured low HDL-C levels and thyroid cancer were 1.243, 1.404, 1.486, and 1.680 (P for trend <0.01), respectively, which were significant even after adjusting for age, sex, lifestyle factors, and metabolic diseases. The subgroup analysis revealed that low HDL-C levels likely had a greater impact on the group of patients with central obesity (P for interaction= 0.062), high blood pressure (P for interaction=0.057), impaired fasting glucose (P for interaction=0.051), and hyperlipidemia (P for interaction=0.126).

CONCLUSION

Repeatedly measured low HDL-C levels can be considered a risk factor for cancer as well as vascular disease. Low HDL-C levels were associated with the risk of thyroid cancer, and this correlation was stronger in a metabolically unhealthy population.

Protection and safety of a repeated dosage of KI for iodine thyroid blocking during pregnancy

In case of nuclear power plant accidents resulting in the release of radioactive iodine (¹³¹I) in large amounts, a single intake of stable iodine is recommended in order to prevent¹³¹I fixation to the thyroid gland. However, in situations of prolonged exposure to¹³¹I (e.g. Fukushima-Daiichi natural and nuclear disaster), repetitive administration of iodine may be necessary to ensure adequate protection, with acceptable safety in vulnerable populations including pregnant women. Here we conducted toxicological studies on adult rats progeny following prolonged exposure to potassium iodide (KI)in utero. Pregnant Wistar rats were treated with 1 mg kg d^-1KI or saline water for 2 or 4 d either between gestation days gestational day (GD) GD 9-12, or GD13-16. Plasma samples from the progeny were tested 30 d post-weaning for clinical biochemistry, thyroid hormones, and anti-thyroid antibody levels. Thyroid and brain were collected for gene expression analysis. The hormonal status was similar for the mothers in all experimental conditions. In the offspring, while thyroid-stimulating hormone and anti-thyroid peroxidase (anti-TPO) antibody levels were similar in all groups, a significant increase of FT3 and FT4 levels was observed in GD9-GD10 and in GD13-GD14 animals treated for 2 d, respectively. In addition, FT4 levels were mildly decreased in 4 d treated GD13-16 individuals. Moreover, a significant decrease in the expression level of thyroid genes involved in iodide metabolism, TPO and apical iodide transporter, was observed in GD13-GD14 animals treated for 2 d. We conclude that repeated KI administration for 2-4 d during gestation did not induce strong thyroid toxicity.

Possible mechanism of bamboo shoots (Bambusa balcooa) induced thyroid disruption - An in vitro study

Endemic goitre and associated iodine deficiency disorders (IDDs) are a major concern in public health even in the period of post salt iodization in many regions. Among others the consumption of cyanogenic plants found responsible for the persistence of such diseases. Bamboo shoots (BS) is one such cyanogenic plant food that caused disruption of certain thyroid hormone synthesizing regulatory element as has already been reported in our earlier study. In this investigation the possible mechanism of thyrocytes disruption along with interruption of thyroid hormone biosynthesis by BS has been worked out. Commonly consumed BS, Bambusa Balcooa Roxb (BBR) water extract was analysed by GC MS; three doses below IC50 were administered to thyrocytes in culture with and without iodine. Expressions of thyroglobulin (Tg), pendrin (PDS) and monocarboxylate transporter 8 (MCT8) were evaluated in thyrocytes with cell cycle analysis, reactive oxygen species (ROS) generation, DNA oxidation and apoptotic regulation through Bax, Bcl-2 and p53. Phytochemical analysis of BBR extract revealed the presence of precursors and metabolic end products of cyanogenic glycosides. Dose dependent decrease in expression of Tg and PDS with concomitant decrease in gene expression of these with MCT8 were observed. Increased ROS, DNA oxidation and associated imbalance were found through increased Bax and p53 with decreased Bcl-2 that perturbed thyrocytes cell cycle. Cyanogenic constituents of BBR generates ROS associated oxidative changes in thyrocytes with DNA damage and oxidation and cell cycle disruption followed by inhibition of thyroid hormone synthesizing regulatory elements; addition of extra iodine showed partial prevention.

Identification of Two Distinct Molecular Subtypes of Non-Invasive Follicular Neoplasm with Papillary-Like Nuclear Features by Digital RNA Counting

BACKGROUND

The follicular variant (FV) of papillary thyroid cancer (PTC) is one of the most common variants of PTC. Clinically, non-infiltrative FVPTC is considered a low-risk variant of PTC, and the non-invasive encapsulated forms of FVPTC represent a group of thyroid tumors with a particularly good prognosis. Consequently, these neoplasms have been very recently reclassified as non-invasive follicular neoplasms with papillary-like nuclear features (NIFTP). From a molecular standpoint, NIFTP appears to be similar to follicular neoplasms. However, only limited data are currently available regarding their gene expression profile.

METHODS

The aim of this study was to identify specific molecular signatures of 26 NIFTPs compared to those of 19 follicular adenomas (FAs) and 18 infiltrative FVPTCs (IFVPTCs). A nanoString custom assay was used to perform mRNA expression analysis. All cases were also genotyped for BRAF, N-, H-, and K-RAS mutations. Samples were grouped on the basis of gene expression profiles by Pearson's correlation and non-negative matrix factorization clustering analysis. Finally, the uncorrelated shrunken centroid machine-learning algorithm was used to classify the samples.

RESULTS

The results revealed distinct expression profiles of FAs and IFVPTCs. NIFTP samples can exhibit different expression profiles, more similar to FAs (FA-like) or to IFVPTCs (IFVPTC-like), and these different expression profiles largely depend on the presence of different mutations (RAS or BRAF).

CONCLUSION

In conclusion, although further validation of the model is required by using a larger group of prospective cases, these data reinforce the hypothesis that IFVPTC-like NIFTPs might represent precursors of IFVPTC.

Meta-analysis of promoter methylation in eight tumor-suppressor genes and its association with the risk of thyroid cancer

Promoter methylation in a number of tumor-suppressor genes (TSGs) can play crucial roles in the development of thyroid carcinogenesis. The focus of the current meta-analysis was to determine the impact of promoter methylation of eight selected candidate TSGs on thyroid cancer and to identify the most important molecules in this carcinogenesis pathway. A comprehensive search was performed using Pub Med, Scopus, and ISI Web of Knowledge databases, and eligible studies were included. The methodological quality of the included studies was evaluated according to the Newcastle Ottawa scale table and pooled odds ratios (ORs); 95% confidence intervals (CIs) were used to estimate the strength of the associations with Stata 12.0 software. Egger's and Begg's tests were applied to detect publication bias, in addition to the "Metatrim" method. A total of 55 articles were selected, and 135 genes with altered promoter methylation were found. Finally, we included eight TSGs that were found in more than four studies (RASSF1, TSHR, PTEN, SLC5A, DAPK, P16, RARβ2, and CDH1). The order of the pooled ORs for these eight TSGs from more to less significant was CDH1 (OR = 6.73), SLC5 (OR = 6.15), RASSF1 (OR = 4.16), PTEN (OR = 3.61), DAPK (OR = 3.51), P16 (OR = 3.31), TSHR (OR = 2.93), and RARβ2 (OR = 1.50). Analyses of publication bias and sensitivity confirmed that there was very little bias. Thus, our findings showed that CDH1 and SCL5A8 genes were associated with the risk of thyroid tumor genesis.

Presence of free triiodothyronine and free thyroxine in thyroid follicles may be correlated with the quick secretion of thyroid hormones under certain physiological conditions

Thyroid cells are polarized and the follicle structure, consisting of follicle epithelial cells, is a prerequisite for thyroid hormone synthesis. However, a reliable in vitro model simulating thyroid function is not currently available. To the best of our knowledge, the present study reports for the first time a simulated follicle by inoculation of human thyroid cells on the filter in a Transwell plate to maintain the polarity of thyroid cells. The iodine uptake was analyzed by arsenic and cerium catalysis spectrophotometry, as well as the secretion of free triiodothyronine (FT3) and free thyroxine (FT4) by direct chemiluminescence. The data showed that thyroid cells growing in the Transwell chamber synthesized and secreted FT3 and FT4, while the monolayer cells directly seeded in the 6-well-plate did not produce these two thyroid hormones. Regarding the iodine uptake, cells in the Transwell chamber demonstrated a markedly higher capability than the monolayer cells. The data proved that the polarity of thyroid cells could be restored using the Transwell plate, which was critical for iodine uptake and thyroid hormone synthesis. The presence of FT3 and FT4 in follicles may be correlated with the quick secretion of thyroid hormones under certain physiological conditions.

Iodide transport and breast cancer

Breast cancer is the second most common cancer worldwide and the leading cause of cancer death in women, with incidence rates that continue to rise. The heterogeneity of the disease makes breast cancer exceptionally difficult to treat, particularly for those patients with triple-negative disease. To address the therapeutic complexity of these tumours, new strategies for diagnosis and treatment are urgently required. The ability of lactating and malignant breast cells to uptake and transport iodide has led to the hypothesis that radioiodide therapy could be a potentially viable treatment for many breast cancer patients. Understanding how iodide is transported, and the factors regulating the expression and function of the proteins responsible for iodide transport, is critical for translating this hypothesis into reality. This review covers the three known iodide transporters - the sodium iodide symporter, pendrin and the sodium-coupled monocarboxylate transporter - and their role in iodide transport in breast cells, along with efforts to manipulate them to increase the potential for radioiodide therapy as a treatment for breast cancer.

Iodine accumulation in sea urchin larvae is dependent on peroxide

Iodine has many important biological functions and its concentrations vary with the environment. Recent research has provided novel insights into iodine uptake mechanisms in marine bacteria and kelp through hydrogen peroxide-dependent diffusion (PDD). This mechanism is distinct from sodium-dependent mechanisms known from vertebrates. In vertebrates, iodine accumulates in the thyroid gland by the action of the apical iodide transporter (AIT) and the sodium/iodide symporter (NIS). Neither of these proteins has, thus far, been identified outside of the chordates, and PDD (as an iodine uptake mechanism) has never been studied in animals. Using (125)I as a marker for total iodine influx, we tested iodine uptake via sodium-dependent transport versus PDD in embryos and larvae of the sea urchin Strongylocentrotus purpuratus. We found that iodine uptake in S. purpuratus is largely independent of NIS/AIT. Instead, we found that uptake is dependent on the presence and production of hydrogen peroxide, indicating that sea urchin larvae use PDD as a mechanism for iodine acquisition. Our data, for the first time, provide conclusive evidence for this mechanism in an animal. Furthermore, our data provide preliminary evidence that sodium-dependent iodine uptake via active transporter proteins is a synapomorphy of vertebrates.

The Effect of Tanespimycin (17-AAG) on Radioiodine Accumulation in Sodium-Iodide Symporter Expressing Cells

PURPOSE

The heat shock protein 90 inhibitor, tanespimycin, is an anticancer agent known to increase iodine accumulation in normal and cancerous thyroid cells. Iodine accumulation is regulated by membrane proteins such as sodium iodide symporter (NIS) and pendrin (PDS), and thus we attempted to characterize the effects of tanespimycin on those genes.

METHODS

Cells were incubated with tanespimycin in order to evaluate (125)I accumulation and efflux ability. Radioiodine uptake and efflux were measured by a gamma counter and normalized by protein amount. RT-PCR were performed to measure the level of gene expression.

RESULTS

After tanespimycin treatment, (125)I uptake was increased by ∼2.5-fold in FRTL-5, hNIS-ARO, and hNIS-MDA-MB-231 cells, but no changes were detected in the hNIS-HeLa cells. Tanespimycin significantly reduced the radioiodine efflux rate only in the FRTL-5 cells. In the FRTL-5 and hNIS-ARO cells, PDS mRNA levels were markedly reduced; the only other observed alteration in the levels of NIS mRNA after tanespimycin treatment was an observed increase in the hNIS-ARO cells.

CONCLUSIONS

These results indicate that cellular responses against tanespimycin treatment differed between the normal rat thyroid cells and human cancer cells, and the reduction in the (125)I efflux rate by tanespimycin in the normal rat thyroid cells might be attributable to reduced PDS gene expression.

The role of epigenetic alterations in papillary thyroid carcinogenesis

Papillary thyroid carcinoma (PTC) accounts for over 80% of all thyroid malignancies. The molecular pathogenesis remains incompletely clarified although activation of the RET fusion oncogenes, and RAS and BRAF oncogenes, has been well characterized. Novel technologies using genome-wide approaches to study tumor genomes and epigenomes have provided great insights into tumor development. Growing evidence shows that acquired epigenetic abnormalities participate with genetic alterations to cause altered patterns of gene expression/function. It has been established beyond doubt that promoter cytosine methylation in CpG islands, and the subsequent gene silencing, is intimately involved in cancer development. These epigenetic events very likely contribute to significant variation in gene expression profiling, phenotypic features, and biologic characteristics seen in PTC. Hypermethylation of promoter regions has also been analyzed in PTC, and most studies have focused on individual genes or a small cohort of genes implicated in tumorigenesis.

Clinical and molecular characteristics of Pendred syndrome

Pendred syndrome is an autosomal recessive disorder defined by sensorineural deafness, goiter and a partial defect in the organification of iodide. It is caused by biallelic mutations in the SLC26A4 gene, which encodes pendrin, a multifunctional anion exchanger. At the level of the inner ear, pendrin is important for the creation of a normal endolymph composition and the maintenance of the endocochlear potential. In the thyroid, pendrin is expressed at the apical membrane of thyroid follicular cells and it appears to be involved in mediating iodide efflux into the lumen and/or maintenance of the follicular pH. Goiter development and hypothyroidism vary among affected individuals and seem to be partially dependent on nutritional iodide intake. In the kidney, pendrin functions as a chloride/bicarbonate exchanger. Elucidation of the molecular basis of Pendred syndrome and the function of pendrin has provided unexpected novel insights into the pathophysiology of the inner ear, thyroid hormone synthesis, and chloride/bicarbonate exchange in the kidney.

Role of iodine in thyroid physiology

Adequate levels of iodine, a trace element variably distributed on the earth, are required for the synthesis of the thyroid hormones thyroxine (T₄) and triiodothyronine (T₃). The iodide cycle consists of a series of transport, oxidation and coupling steps in thyroid follicular cells to produce thyroid hormone. The sodium/iodide symporter (NIS) transports iodide into the thyrocyte. Competitive inhibitors of NIS, such as perchlorate and thiocyanate, can decrease the entrance of iodide into the follicular cell. Pendrin is the primary protein that is responsible for iodide efflux out of the thyrocyte and into the follicular lumen. T₄ is deiodinated in target tissues to produce the active form of thyroid hormone, T₃, and other metabolites. Exposure to excessive iodine or chronic iodine deficiency may result in various clinical disorders. The Wolff-Chaikoff effect and Jöd-Basedow phenomenon describe mechanisms of thyroid autoregulation and dysregulation, respectively, during iodine excess. Population studies have determined that iodine deficiency exists in approximately 38% of the world's population, is the leading cause of preventable mental retardation, and is of particular concern to women and their infants. Finally, the unique role of iodine utilization in thyroid physiology has applications in many important clinical areas.

Benefits of thyrotropin suppression versus the risks of adverse effects in differentiated thyroid cancer

BACKGROUND

Despite clinical practice guidelines for the management of differentiated thyroid cancer (DTC), there are no recommendations on the optimal serum thyrotropin (TSH) concentration to reduce tumor recurrences and improve survival, while ensuring an optimal quality of life with minimal adverse effects. The aim of this review was to provide a risk-adapted management scheme for levothyroxine (L-T4) therapy in patients with DTC. The objective was to establish which patients require complete suppression of serum TSH levels, given their risk of recurrent or metastatic DTC, and how potential adverse effects on the heart and skeleton, induced by subclinical hyperthyroidism, in concert with advanced age and comorbidities, may influence the degree of TSH suppression.

SUMMARY

A risk-stratified approach to predict the rate of recurrence and death from thyroid cancer was based on the recently revised American Thyroid Association guidelines. A stratified approach to predict the risk from the adverse effects of L-T4 was devised, taking into account the age of the patient, as well as the presence of preexisting cardiovascular and skeletal risk factors that might predispose to the development of long-term adverse cardiovascular or skeletal outcomes, particularly increased heart rate and left ventricular mass, atrial fibrillation, and osteoporosis. Nine potential patient categories can be defined, with differing TSH targets for both initial and long-term L-T4 therapy.

CONCLUSION

Before deciding on the degree of TSH suppression during initial and long-term L-T4 treatment in patients with DTC, it is necessary to consider the aggressiveness of DTC, as well as the potential for adverse effects induced by iatrogenic subclinical hyperthyroidism. More aggressive TSH suppression is indicated in patients with high-risk disease or recurrent tumor, whereas less aggressive TSH suppression is reasonable in low-risk patients. In patients with high-risk DTC and an equally high risk of adverse effects, long-term treatment with L-T4 therapy should be individualized and balanced against the potential for adverse effects. In patients with an intermediate risk for thyroid cancer recurrence and a high risk of adverse effects of therapy, the degree of TSH suppression should be reevaluated during the follow-up period. Normalization of serum TSH is advisable for long-term treatment of disease-free elderly patients with DTC and significant comorbidities.

[Pendred's syndrome: a cause of goiter associated with deafness]

Pendred's syndrome is an autosomal recessive disorder leading to congenital sensorineural hearing loss and a variable degree of goiter due to reduced iodine organification. The cause of this disease is dysfunction of an anion transporter protein located on the apical membrane of thyrocytes, called pendrin, which is also found in the kidney and cochlea. Molecular analysis of the gene is useful to identify other affected family members and provide proper genetic advice and early diagnosis in descendants. We present the cases of two siblings with sensorineural deafness who were diagnosed with Pendred's syndrome as adults because one of them consulted for goiter.

Reciprocal immunohistochemical expression of sodium/iodide symporter and hexokinase I in primary thyroid tumors with synchronous cervical metastasis

OBJECTIVES/HYPOTHESIS

Sodium/iodide symporter (NIS) is a glycoprotein which is related to the concentration of radioiodine in thyroid cancer. Glucose transporter-1 (Glut-1) and hexokinases (HK) are glycoproteins related to glucose metabolism (i.e., uptake and phosphorylation) in various cancers. The aim of this study was to determine the immunohistochemical patterns of expression and mutual relationships between NIS, Glut-1, HK I, and HK II in primary thyroid tumors and synchronous cervical metastatic tumors.

STUDY DESIGN

Retrospective experimental study.

METHODS

Nine cases of follicular carcinomas, sixteen cases of papillary carcinomas, and four cases of anaplastic carcinomas were included. The immunohistochemical staining intensities were categorized (scored) as negative (0), weak (1), positive (2), or strongly positive (3) and dichotomized as a negative/weak (scores 0 or 1) or positive expression (scores 2 or 3).

RESULTS

NIS had a positive expression in good prognostic types of thyroid carcinomas, such as follicular carcinomas, more often, while having a positive expression in poor prognostic types of thyroid carcinomas, such as anaplastic carcinomas (P = .033) less often; HK I had the opposite pattern of expression (P = .033). Primary thyroid tumors and corresponding synchronous cervical metastatic tumors had a similar pattern of expression for NIS, HK I, and HK II. NIS had a reciprocal relationship with HK I as compared to Glut-1 with respect to staining intensity on each primary tumor (P = .040).

CONCLUSION

Reciprocal staining pattern of NIS and HK I on primary tumors is related to the staining pattern of NIS and HK I on synchronous as well as occult cervical metastatic tumors.

Expression of iodine metabolism genes in human thyroid tissues: evidence for age and BRAFV600E mutation dependency

CONTEXT

Children present a higher susceptibility to developing thyroid cancer after radioiodine exposure and also a higher frequency of functional metastases than adults.

OBJECTIVE

To assess the mRNA expression of the sodium/iodide (Na(+)/I(-)) symporter (NIS), the Pendred syndrome gene (PDS), thyroperoxidase (TPO), thyroglobulin (Tg) and TSH receptor (TSH-R) in normal thyroid tissues (NTTs) and papillary thyroid carcinomas (PTCs) among different age groups.

METHODS

Analysis included 59 samples: 21 NTTs and 38 PTCs, of which 21 were the classic type (CPTC) and 17 the follicular variant (FVPTC). Patients were divided into three age groups: I (n = 16) 5-21 years, II (n = 13) 22-59 years, and III (n = 10) 60-91 years. The relative mRNA expression of the five target genes was determinate by quantitative reverse transcription polymerase chain reaction (QRT-PCR).

RESULTS

Expression of all genes was significantly higher in NTTs than in PTCs, and it was not age dependent in the NTT group. Among PTCs, the mean expression of PDS, TPO and TSH-R was significantly lower in group II than in group I. PDS, TPO and Tg expression was significantly lower in classic PTCs than in FVPTCs. The difference was related to a higher frequency of the BRAF(V600E) mutation in the former group.

CONCLUSIONS

The finding of higher PDS, TPO and TSH-R mRNA expression in paediatric vs. adult primary tumour tissues supports the hypothesis that this might contribute to the increased functional activity of metastases in the paediatric group. The finding that mRNA expression of the target genes in NTT was not age dependent does not provide an explanation for the higher susceptibility in the paediatric group.

Pendred syndrome and iodide transport in the thyroid

Pendred syndrome is an autosomal recessive disorder characterized by sensorineural hearing impairment, presence of goiter, and a partial defect in iodide organification, which may be associated with insufficient thyroid hormone synthesis. Goiter development and development of hypothyroidism are variable and depend on nutritional iodide intake. Pendred syndrome is caused by biallelic mutations in the SLC26A4 gene, which encodes pendrin, a transporter of chloride, bicarbonate and iodide. This review discusses the controversies surrounding the potential role of pendrin in mediating apical iodide efflux into the lumen of thyroid follicles, and discusses its functional role in the kidney and the inner ear.

Suppression of BRAF/MEK/MAP kinase pathway restores expression of iodide-metabolizing genes in thyroid cells expressing the V600E BRAF mutant

PURPOSE

The V600E BRAF mutant plays an important role in the pathogenesis of papillary thyroid cancer (PTC) and is associated with loss of expression of thyroid iodide-metabolizing genes. This study was done to investigate the restorability of expression of these genes by suppressing the BRAF/extracellular signal-regulated kinase kinase (MEK)/mitogen-activated protein (MAP) kinase pathway in V600E BRAF-harboring thyroid cells and to explore the mechanisms involved.

EXPERIMENTAL DESIGN

We used inducible expression of V600E BRAF, small interfering RNA transfection, and MEK-specific inhibitor to alter the MAP kinase pathway activities and subsequently examined the changes in expression, promoter activities, and methylation status of thyroid genes.

RESULTS

MEK inhibitor U0126 or cessation of V600E BRAF expression in PCCL3 cells restored expression of thyroid genes silenced by induced expression of V600E BRAF. U0126 also restored the expression of these genes in V600E BRAF-harboring PTC-derived NPA cells. Knockdown of BRAF by specific small interfering RNA restored expression of some of these genes in NPA cells. Luciferase reporter assay using thyroid-stimulating hormone receptor gene as a model showed that the promoter activity was modulated by the MAP kinase pathway. Promoter methylation in association with DNA methyltransferase expression played a role in gene silencing by MAP kinase pathway in NPA cells.

CONCLUSIONS

We showed the restorability of expression of thyroid iodide-metabolizing genes silenced by V600E BRAF, and linked this process to gene methylation in PTC cells. The results provide clinical implications that therapeutic targeting at the BRAF/MEK/MAP kinase pathway may be a good approach in restoring thyroid gene expression for effective radioiodine therapy for BRAF mutation-harboring PTC.

Transport of free 211At and 125I- in thyroid epithelial cells: effects of anion channel blocker 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid on apical efflux and cellular retention

INTRODUCTION

Astatine ((211)At; alpha-emitter; t(1/2)=7.21 h) shares several features with its halogen neighbour iodine. In the present study, we investigated whether 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) can be used to increase the cellular retention time of (211)At and radioiodide in thyroid epithelial cells.

METHODS

The transepithelial transport and cellular uptake of (211)At and (125)I(-) were studied simultaneously in porcine thyrocytes cultured in bicameral chambers. The cells were prestimulated with thyroid-stimulating hormone (TSH) or epidermal growth factor (EGF) for 48 h. In addition, the acute effects of DIDS and forskolin were investigated.

RESULTS

The transepithelial transport of both radionuclides was stimulated by TSH and down-regulated by EGF. DIDS rapidly reduced the efflux and increased the cellular content of (125)I(-) in control and TSH-stimulated cells, whereas DIDS had no effect on (125)I(-) transport in EGF-treated cells. DIDS blocked the (211)At efflux only in TSH-stimulated cells. Unexpectedly, DIDS caused an accelerated efflux of (211)At in both control and EGF-stimulated cells and, furthermore, reduced the cellular content of (211)At in the EGF-stimulated cultures. DIDS had no effect on the forskolin-induced efflux of the two radionuclides.

CONCLUSIONS

The magnitude of thyroidal (211)At uptake and efflux is similar to that of (125)I(-), strongly dependent on the functional activity of the cells. However, (211)At efflux likely involves several permeating mechanisms with different sensitivity to DIDS, which are at least partly not shared by (125)I(-). The results suggest that anion channel blockage is potentially useful to increase the absorbed dose from both (211)At and radioiodine in NIS-expressing tumours.

Defects in iodide metabolism in thyroid cancer and implications for the follow-up and treatment of patients

The two major steps of iodine metabolism--uptake and organification--are altered in thyroid cancer tissues. Organification defects result in a rapid discharge of radioiodine from thyroid cells, a short effective half-life of iodine, and a low rate of thyroid hormone synthesis. These defects are mainly due to decreased expression of functional genes encoding the sodium-iodide symporter and thyroid peroxidase and could result in a low radiation dose to thyroid cancer cells. TSH stimulation that is achieved with injections of recombinant human TSH, or long-term withdrawal of thyroid hormone treatment increases iodine-131 uptake in two-thirds of patients with metastatic disease and increases thyroglobulin production in all patients with metastases, even in the absence of detectable uptake. Serum thyroglobulin determination obtained following TSH stimulation and neck ultrasonography is the most sensitive combination for the detection of small tumor foci. Radioiodine treatment is effective when a high radiation dose can be delivered (in patients with high uptake and retention of radioiodine) and when tumor foci are sensitive to the effects of radiation therapy (younger patients, with a well-differentiated tumor and/or with small metastases). The other patients rarely respond to radioiodine treatment, and when progression occurs, other treatment modalities should be considered. Novel strategies are currently being explored to restore iodine uptake in cancer cells that are unable to concentrate radioiodine.

Gene methylation in thyroid tumorigenesis

Aberrant gene methylation plays an important role in human tumorigenesis, including thyroid tumorigenesis. Many tumor suppressor genes are aberrantly methylated in thyroid cancer, and some even in benign thyroid tumors, suggesting a role of this epigenetic event in early thyroid tumorigenesis. Methylation of some of these genes tends to occur in certain types of thyroid cancer and is related to specific signaling pathways. For example, methylation of PTEN and RASSF1A genes occurs mostly in follicular thyroid cancer, and its tumorigenic role may be related to the phosphatidylinositol 3-kinase/Akt signaling pathway, whereas methylation of genes for tissue inhibitor of metalloproteinase-3, SLC5A8, and death-associated protein kinase occurs in papillary thyroid cancer and is related to the BRAF/MAPK kinase/MAPK pathway. Methylation of thyroid-specific genes, such as those for sodium/iodide symporter and thyroid-stimulating hormone receptor, is also common in thyroid cancer. Although its tumorigenic role is not clear, methylation, and hence silencing, of these thyroid-specific genes is a cause for the failure of clinical radioiodine treatment of thyroid cancer. Unlike gene methylation, histone modifications have been relatively poorly investigated in thyroid tumors. Future studies need to emphasize the mechanistic aspects of these two types of epigenetic alterations to uncover new molecular mechanisms in thyroid tumorigenesis and to provide novel therapeutic targets for thyroid cancer.

Functional characterization of human thyroid tissue with immunohistochemistry

Immunohistochemistry provides insights in the expression of functional proteins and of their localization in normal thyroid tissue and in thyroid diseases. In hyperfunctional thyroid tissues, staining for sodium/iodide symporter (NIS), pendrin, thyroid peroxidase (TPO), and thyroglobulin (Tg) is increased. In hypofunctioning thyroid tissues, NIS staining is markedly decreased; in benign hypofunctioning adenomas, the expression of the other functional proteins is unmodified or slightly decreased, whereas their expression is profoundly decreased or absent in differentiated thyroid carcinoma.

Association of aberrant methylation of tumor suppressor genes with tumor aggressiveness and BRAF mutation in papillary thyroid cancer

The role of aberrant tumor suppressor gene methylation in the aggressiveness of papillary thyroid cancer (PTC) has not been documented. By showing promoter methylation-induced gene silencing in PTC-derived cell lines, we first demonstrated the functional consequence of methylation of several recently identified tumor suppressor genes, including those for tissue inhibitor of metalloproteinase-3 (TIMP3), SLC5A8, death-associated protein kinase (DAPK) and retinoic acid receptor beta2 (RARbeta2). We then investigated the role of methylation of these genes in the aggressiveness of PTC by examining the relationship of their aberrant methylation to clinicopathological characteristics and BRAF mutation in 231 primary PTC tumors. Methylation of TIMP3, SLC5A8 and DAPK was significantly associated with several aggressive features of PTC, including extrathyroidal invasion, lymph node metastasis, multifocality and advanced tumor stages. Methylation of these genes was also significantly associated with BRAF mutation in PTC, either individually or collectively in various combinations. Methylation of these genes, either individually or collectively, occurred more frequently in more aggressive classical and tall-cell PTC subtypes than in less aggressive follicular-variant PTC, with the latter known to infrequently harbor BRAF mutation. Several other tumor suppressor genes investigated were not methylated. These results suggest that aberrant methylation and hence silencing of TIMP3, SLC5A8, DAPK and RARbeta2, in association with BRAF mutation, may be an important step in PTC tumorigenesis and progression.

From the molecular characterization of iodide transporters to the prevention of radioactive iodide exposure

In the event of a nuclear reactor accident, the major public health risk will likely result from the release and dispersion of volatile radio-iodines. Upon body exposure and food ingestion, these radio-iodines are concentrated in the thyroid, resulting in substantial thyroidal irradiation and accordingly causing thyroid cancers. Stable potassium iodide (KI) effectively blocks thyroid iodine uptake and is thus used in iodide prophylaxis for reactor accidents. The efficiency of KI is directly related to the physiological inhibition of the thyroid function in the presence of high plasma iodide concentrations. This regulation is called the Wolff-Chaikoff effect. However, to be fully effective, KI should be administered shortly before or immediately after radioiodine exposure. If KI is provided only several hours after exposure, it will elicit the opposite effect e.g. lead to an increase in the thyroid irradiation dose. To date, clear evaluation of the benefit and the potential toxicity of KI administration remain difficult, and additional data are needed. We outline in this review the molecular characterization of KI-induced regulation of the thyroid function. Significant advances in the knowledge of the iodide transport mechanisms and thyroid physiology have been made. Recently developed molecular tools should help clarify iodide metabolism and the Wolff-Chaikoff effect. The major goals are clarifying the factors which increase thyroid cancer risk after a reactor accident and improving the KI administration protocol. These will ultimately lead to the development of novel strategies to decrease thyroid irradiation after radio-iodine exposure.

Na(+)/monocarboxylate transport (SMCT) protein expression correlates with survival in colon cancer: molecular characterization of SMCT

We report an extensive characterization of the Na(+)/monocarboxylate transporter (SMCT), a plasma membrane protein that mediates active transport of monocarboxylates such as propionate and nicotinate, and we show that SMCT may play a role in colorectal cancer diagnosis. SMCT, the product of the SLC5A8 gene, is 70% similar to the Na(+)/I(-) symporter, the protein that mediates active I(-) uptake in the basolateral surface of thyrocytes and other cells. SMCT was reported in the apical surface of thyrocytes and formerly proposed also to transport I(-) and was called the apical I(-) transporter. However, it is now clear that SMCT does not transport I(-). Here we demonstrate a high-affinity Na(+)-dependent monocarboxylate transport system in thyroid cells, which is likely to be SMCT. We show that, whereas thyroidal Na(+)/I(-) symporter expression is thyroid-stimulating hormone (TSH)-dependent and basolateral, SMCT expression is TSH-independent and apical not only in the thyroid but also in kidney and colon epithelial cells and in polarized Madin-Darby canine kidney cells. We determine the kinetic parameters of SMCT activity and show its inhibition by ibuprofen (K(i) = 73 +/- 9 microM) in Xenopus laevis oocytes. SMCT was proposed to be a tumor suppressor in colon cancer. Significantly, we show that higher expression of SMCT in colon samples from 113 colorectal cancer patients correlates with longer disease-free survival, suggesting that SMCT expression may be a favorable indicator of colorectal cancer prognosis.

Histochemical demonstration of a Na(+)-coupled transporter for short-chain fatty acids (slc5a8) in the intestine and kidney of the mouse

Short-chain fatty acids in the intestinal lumen affect colonic cell proliferation as well as function as an energy source for intestinal epithelial cells. A novel transporter of monocarboxylates, Slc5a8, is expressed abundantly in the colon, where it may participate in the Na(+)-coupled absorption of short-chain fatty acids produced by bacterial fermentation of dietary fiber. The present study examined the cellular localization of Slc5a8 in the murine gastrointestinal tract and kidney by in situ hybridization and immunohistochemistry. The hybridization signals were recognized in the terminal ileum and whole length of the large intestine, and were especially intense in the distal colon and rectum. The immunoreactivity of Slc5a8 was restricted to the striated border (the brush border) of enterocytes, and was not present in goblet cells, Paneth cells, or lamina propria cells. In the kidney, proximal tubules of both the cortex and the outer stripe of the outer medulla intensely expressed Slc5a8 mRNA, while the distal portions, including the loop of Henle, lacked the signals. The renal Slc5a8 immunoreactivity was localized only in the brush border of proximal tubules, not along the basolateral membrane. Thyroid follicular cells were immunoreactive for Slc5a8, with predominant labeling on the apical membrane. No other organs, including the esophagus, stomach, liver, pancreas, and salivary glands contained any notable signals of Slc5a8. These findings on the cellular and subcellular localization of Slc5a8 under normal conditions are helpful for understanding the physiological and pathological roles of Slc5a8.

Modulation of thyroid-specific gene expression in normal and nodular human thyroid tissues from adults: an in vivo effect of thyrotropin

CONTEXT

Evidence from in vitro studies or animal models has shown that TSH affects thyrocytes by thyroid-specific expression modulation.

OBJECTIVE

The objective of our study was to analyze the role of TSH in human thyroid gene expression in vivo.

DESIGN/SETTING

Thirty-nine normal thyroid tissues were collected at the same center.

STUDY SUBJECTS

Patients were divided into two groups based on serum TSH levels: 17 with normal TSH levels (1-4 mU/liter; group 1) and 22 with TSH levels below 0.5 mU/liter (group 2).

INTERVENTION

Group 2 underwent thyroidectomy after suppressive L-T4 therapy.

MAIN OUTCOME MEASURES

mRNA levels of thyroid genes such as sodium/iodide symporter (NIS), apical iodide transporter, pendrin, thyroglobulin, thyroperoxidase, TSH receptor, paired box transcription factor 8, and thyroid transcription factor-1 were evaluated by quantitative PCR.

RESULTS

The reduction of TSH stimulation causes decreases in NIS and apical iodide transporter gene expression in normal tissues and more limited reductions in thyroglobulin, thyroperoxidase, and paired box transcription factor 8, but it has no significant effect on TSH receptor, pendrin, or thyroid transcription factor-1. Comparison of NIS levels in normal and nodular tissues from the same patient confirmed that it is differentially expressed in nodules only in the presence of normal TSH (P < 0.01). In patients with suppressed TSH, nodular NIS levels were similar to those in normal tissues.

CONCLUSIONS

Our data represent the first demonstration in human thyroid tissues that TSH contributes to the regulation of thyrocyte differentiation by modulating thyroid gene levels. It exerts a particularly important effect on the transcription of NIS, which becomes very low after prolonged TSH suppression.

DARPP-32 (dopamine and 3',5'-cyclic adenosine monophosphate-regulated neuronal phosphoprotein) is essential for the maintenance of thyroid differentiation

Coordination of events leading to differentiation is mediated by the concerted action of multiple signal transduction pathways. In general, the uncoupling of mechanisms linking differentiation to cell cycle exit is a hallmark of cancer, yet the identity and regulation of molecules integrating signal transduction pathways remains largely unknown. One notable exception is DARPP-32 (dopamine and cAMP-regulated neuronal phosphoprotein, molecular mass, 32 kDa), a third messenger that integrates multiple signaling pathways in the brain. Thyroid cells represent an excellent model for understanding the coupling of signal transduction pathways leading to both proliferation and differentiation. The cooperative action of IGF-I and TSH together, but not alone, enable thyroid cells to proliferate while maintaining their differentiated state. How signaling downstream from these molecules is integrated is not known. Here we show that DARPP-32 expression is targeted by TSH and IGF-I in thyrocytes. Significantly, dedifferentiated, tumoral, or Ras-transformed thyrocytes fail to express DARPP-32 whereas short interfering RNA-mediated silencing of DARPP-32 expression in normally differentiated thyroid cells results in loss of differentiation markers such as thyroid transcription factor 1, Pax8, thyroglobulin, and the Na/I symporter. Consistently, DARPP-32 reexpression in ras-transformed cells results in reactivation of the otherwise silent thyroglobulin and thyroperoxidase promoter. Thus, DARPP-32 is critical for the maintenance of thyroid differentiation by TSH and IGF-I, and loss of DARPP-32 expression may be a characteristic of thyroid cancer. Our results also raise the possibility that DARPP-32 may play a similar role in the maintenance of differentiation of a range of other cell types.

Morphological changes of follicular cell basal borders and basement membranes in benign and malignant nodular lesions of the thyroid gland: an ultrastructural study

Microfollicular nodular lesions of the thyroid gland may represent a differential diagnosis problem. Firstly, nodular areas of follicular hyperplasia have to be distinguished from follicular adenomas. On the other hand, nodular microfollicular areas exhibiting large pale nuclei, occasionally found in hyperplastic nodules and follicular adenomas, must be discriminated from latent papillary carcinomas with predominant follicular architecture. The diagnosis of follicular carcinoma still requires the detection of vascular and/or capsular microinvasion. A more refined study was planned to search for additional descriptors useful for diagnosis The authors report the results of an ultrastructural investigation carried out on 220 thyroid nodular lesions and 50 specimens of macroscopically nonnodular glands. An infolding arrangements of the thyreocyte basal border (TBB) and follicular basement membrane (FBM) was demonstrated in 50/50 nonnodular thyroid tissue specimens and 53/67 (79.1%) hyperplastic nodular lesions (p<.005). A linear arrangement of the TBB and FBM was found in 85/121 (70.2%) follicular adenomas and in 32/32 differentiated carcinomas (p<.001). In the last group, 12/32 (37.5%) cases showed focal discontinuities of FBM. In conclusion, the benign thyroid nodules show a prevalently infolding arrangements of TBBs, whereas the majority of proliferative lesions display a linear morphology. In absence of an infiltrating pattern there is no morphological evidence of discriminating potentially malignant vs. benign lesions. The linear distribution of TBBs and FBMs places the case in a group of borderline lesions that involve a more careful postsurgery investigation.

Biological functions of SLC5A8, a candidate tumour suppressor

SLC5A8 is a candidate tumour suppressor gene that is silenced in colon cancer, gastric cancer and possibly other cancers in humans. This gene codes for a transporter belonging to the Na+/glucose co-transporter gene family (SLC5). The cancer-associated silencing of the gene involves hypermethylation of CpG islands present in exon 1 of the gene. SLC5A8 is expressed in colon, ileum, kidney and thyroid gland. The protein coded by the gene mediates the Na+-coupled and electrogenic transport of a variety of monocarboxylates, including short-chain fatty acids, lactate and nicotinate. It may also transport iodide. The normal physiological function of this transporter in the intestinal tract and kidney is likely to facilitate the active absorption of short-chain fatty acids, lactate and nicotinate. One of the short-chain fatty acids that serves as a substrate for SLC5A8 is butyrate. This fatty acid is an inhibitor of histone deacetylases and is known to induce apoptosis in a variety of tumours including colonic tumour. Since butyrate is produced in the colonic lumen at high concentrations by bacterial fermentation of dietary fibre, we speculate that the ability of SLC5A8 to mediate the entry of this short-chain fatty acid into colonic epithelial cells underlies the potential tumour suppressor function of this transporter.

Expression of slc5a8 in Kidney and Its Role in Na+-coupled Transport of Lactate

We report here on the expression of slc5a8 in kidney and its relevance to Na(+)-coupled reabsorption of lactate. slc5a8 is the murine ortholog of SLC5A8, a candidate tumor suppressor gene, which we recently cloned from human intestine and demonstrated its functional identity as a Na(+)-coupled transporter for short-chain fatty acids and lactate. The slc5a8 cDNA, cloned from mouse kidney, codes for a protein consisting of 611 amino acids. When expressed heterologously in mammalian cells or Xenopus oocytes, slc5a8 mediates Na(+)-coupled electrogenic transport of lactate/pyruvate as well as short-chain fatty acids (e.g. acetate, propionate, and butyrate). The Na+/fatty acid stoichiometry varies depending on the fatty acid substrate (2:1 for lactate and 4:1 for propionate). This phenomenon of variable Na+/substrate stoichiometry depending on the fatty acid substrate is also demonstrable with human SLC5A8. In situ hybridization with sagittal sections of mouse kidney demonstrates abundant expression of the transcripts in the cortex as well as the medulla. Brush border membrane vesicles prepared from rabbit kidney are able to transport lactate in a Na(+)-coupled manner. The transport process exhibits the overshoot phenomenon, indicating uphill lactate transport in response to the transmembrane Na+ gradient. The Na(+)-coupled lactate transport in these membrane vesicles is inhibitable by short-chain fatty acids. We conclude that slc5a8 is expressed abundantly in the kidney and that it plays a role in the active reabsorption of lactate. slc5a8 is the first transporter known to be expressed in mammalian kidney that has the ability to mediate the Na(+)-coupled reabsorption of lactate.